Aren't these values provided by the layer-3 and provider extensions?

@jamiehannaford these values are retrievable via /v2.0/networks, why not also map them to Network model?

Besides, what is the general strategy for sdk to cover extensions?

@haphan All extensions code should live under the extensions dir for that service, for example here. I wanted to do this so that we don't make any assumptions about users using a specific extension and mixing that code with the core service.

Even if the extension modifies a core URL endpoint (e.g. /networks) I still think we should separate but could have the extension resource class (ExtNetwork) extend the core resource (Network). We could even use the core Api and Params classes if the operations are identical, but at least we'd have separation.